End stage renal disease (ESRD) is a condition characterized by a disproportionately high risk of premature cardiovascular (CV) morbidity and mortality, due in part to low-grade chronic inflammation. Efforts to reduce the inflammatory load in ESRD have been largely unsuccessful. Microbial translocation into the bloodstream can occur via different routes in ESRD, including contaminated dialysate, dialysis catheter, and impaired intestinal mucosa, and can be a potential cause of chronic inflammation. While bacterial endotoxins have been extensively studied among microbial components identifiable in the blood, recent advances in microbial DNA sequencing have allowed the identification of highly diverse microbial communities in the systemic circulation (a.k.a. circulating microbiome), even in the absence of overt infection. In addition, both quantitative and qualitative changes in circulating microbiome have been shown to be associated with conditions linked to chronic inflammation, such as CV disease, potentially through their immunostimulatory, atherogenic, and cardiotoxic properties. These results suggest that the circulating microbiome may contribute to the high rates of chronic inflammation and premature mortality in ESRD. Nevertheless, no work to date has described the nature of the circulating microbiome and its longitudinal relationships with clinical outcomes in ESRD. The central hypothesis of this proposal is that microorganisms including bacteria, archaea and fungi are chronically present in the systemic circulation of patients with ESRD and contribute to the excess risk of chronic inflammation and premature mortality. The objective of this project is to characterize the circulating microbiome in ESRD and to investigate its associations with premature mortality. The long-term goal is to elucidate the biological mechanisms underlying the relationships between circulating microbiome and premature mortality in ESRD. To test our hypothesis, we propose to analyze longitudinally collected blood samples from a nationwide prospective cohort of 978 prevalent hemodialysis (HD) patients, with the following specific aims: 1) Determine the levels, composition, and temporal changes of the circulating microbiome by applying metagenomic sequencing techniques; 2) Examine the association of circulating microbiome with all-cause and CV mortality, and also determine if inflammation acts as a mediator of this association; and 3) Identify clinical factors that are associated with mortality-related circulating microbiome in HD patients by utilizing both traditional regression analyses and innovative data-driven techniques to discover potentially modifiable risk factors. The findings of our proposed study will enhance our understanding of the characteristics and roles of circulating microbiome in HD patients and pave the way for the future development of novel diagnostic and prognostic biomarkers and target-driven therapeutic strategies to reduce excess risk of mortality in ESRD.